In recent gas turbine combustors, a premixed combustion method is used from a standpoint of environmental protection because the premixed combustion method is more advantageous for a reduction of thermal NOx. The premixed combustion method is for premixing fuel and excessive air and burning the premixed fuel, which can easily reduce NOx, because the fuel burns under a diluted condition in all spaces in the combustor. The premixing combustor in a gas turbine is explained and a premixing nozzle used heretofore is explained as well.
FIG. 14 shows a premixing combustor and a premixing nozzle in a gas turbine used heretofore. A combustion nozzle block 505 is provided in a combustor casing 600, with a certain space from the combustor casing, and a pilot corn 60 for forming diffusion flame is provided in the central part of the combustion nozzle block 505. This combustion nozzle block 505 is inserted in an inner cylinder 515 of a combustion chamber. The pilot corn 60 forms the diffusion flame by allowing a pilot fuel supplied from a pilot fuel supply nozzle 62 to react with combustion air supplied from a compressor (not shown).
Though not clear from FIG. 14, eight premixing nozzles 820 for forming premixed flame are provided around the pilot corn 60. Swirler blades 320 for swirling the combustion air are attached inside a nozzle body 10. The swirler blades 320 swirl the combustion air fed from the compressor (not shown) to produce a rotational flow in the combustion air, thereby mixing the fuel and the combustion air. A hub 120 for holding a fuel nozzle shaft 220, described later, is fitted in the central part of the swirler blades 320.
The fuel nozzle shaft 220 for supplying the fuel is inserted into the hub 120, and is supported substantially at the center of the nozzle body 10 by the swirler blades 320 and the hub 120. The fuel nozzle shaft 220 is provided with hollow gas fuel supply blades 29, and the gas fuel fed from a fuel supply path provided in the fuel nozzle shaft 220 is guided to the inside of the gas fuel supply blades 29. The gas fuel is then supplied from gas fuel supply holes 49 provided on the sides of the gas fuel supply blades 29 into the nozzle body 10.
In the process that the fuel supplied to the nozzle body 10 flows through inside of the body to the downstream, the fuel is sufficiently mixed with the combustion air swirled by the swirler blades 320 to form a premixed gas. This premixed gas is injected from an outlet 10a of the nozzle body 10 into the inner cylinder 515 of the combustion chamber, and ignited by high temperature combustion gas exhausted from the diffusion flame to form premixed gas combustion flame. High temperature and high pressure combustion gas is exhausted from the premixed gas flame, and is guided to a first stage nozzle of a turbine through a combustor tailpipe (not shown).
The premixing nozzle 820 used heretofore in the premixing combustor is for promoting mixture of the fuel and the combustion air by swirling the combustion air by the swirler blades 320. However, when the combustion air is swirled by the swirler blades 320, the flow velocity near the center of the nozzle body 10 decreases due to a centrifugal force derived from the swirls (see FIG. 3(a)). When the flow velocity decreases near the center of the nozzle body 10, the premixed gas tends to flow backward to the part where the flow velocity is low. As a result, flashback occurs, and the nozzle body 10 and the fuel nozzle shaft 220 may be burnout. This damage by burning shortens the life of the premixing nozzle, and hence repair or replacement is required frequently, causing a problem in that labor hour is required for the maintenance.
It is an object of the present invention to solve at least the problems in the conventional technology.